It is well known that electromagnetic energy can be utilized for heating foodstuff or other lossy dielectric materials. The foodstuff or other materials are placed in a cooking cavity of a microwave oven and are exposed to electromagnetic energy that is supplied by a suitable source, e.g., a magnetron. After a relatively short period of time during which the foodstuff is subjected to electromagnetic energy, heat will be generated in the foodstuff to accomplish the desired cooking of the foodstuff.
In a microwave oven, an ideal system for exciting the cooking cavity with microwave energy would evenly distribute the microwave energy across those portions of the cavity in which the food is located. Since food is normally located in only a limited area of the oven it is desirable to maximize the energy in the portion of the cavity in which the food is to be located.
Microwave ovens have employed numerous types of feed and distribution systems in an attempt to maximize the energy supplied to the food. For example, in British Patent No. 1,407,852, there is disclosed a microwave oven which utilizes "near field" effects of electromagnetic radiation to heat foods. In this method, the food is maintained in close proximity with a radiation element, a proximity preferably less than one wavelength of the exciting electromagnetic energy. Also, in U.S. Pat. No. 3,810,248, a microwave apparatus is described in which food is placed in a container over slotted openings in a waveguide. The food is heated directly by the microwave energy exiting the waveguide, and indirectly by a radiation absorbing layer in the container that is contact with the food. In addition, U.S. Pat. No. 3,851,133 discloses a microwave oven which includes an antenna chamber, with an antenna in the form of radially extending arms rotating around a common axis mounted therein, disposed adjacent to the cooking cavity with microwave energy being introduced into the cavity through radiation slots disposed on the side of the cavity adjacent the antenna chamber. Also, U.S. Pat. No. 4,019,009 discloses a microwave oven which heats food by subjecting it to a microwave field generated by a surface wave transmission line comprising a slotted wall.
Other known methods for providing a more uniform distribution of microwave energy rely on the use of rotating devices within the cooking cavity, such as a mode stirrer having blades and being driven by a motor cyclically, a rotating food tray, or a rotating antenna. Each of the methods described above has certain drawbacks, particularly if installed in a hybrid oven which combines microwave heating with conventional oven techniques, where the internal temperature of the oven cavity is normally approximately 500 degrees F, such as is more particularly described in U.S. Pat. Nos. 5,254,823, 5,451,751 and 5,558,793.
A particular problem facing many microwave ovens relates to loading effects caused by the size of the foodstuff installed in the cooking cavity. When a small item is placed in the cavity, or especially when the cavity is empty, and the oven is operated, the microwave energy can reflect back to the magnetron, where it is dissipated as heat and can eventually damage the magnetron. A number of methods have been used to prevent such damage to the magnetron. One such method involved placing a thermostat in proximity to the anode of the magnetron to detect the temperature at the magnetron. A control circuit cut off power to the magnetron when the temperature reached a point at which damage would occur. However, in this method the magnetron can still be stressed by relatively high temperatures. Another method, disclosed in U.S. Pat. No. 3,527,915, provides a no-load sensing device that is mounted within the cooking cavity to cut off the magnetron when a specified temperature is reached within the device. In addition, U.S. Pat. No. 5,451,751 discloses a microwave oven which utilizes a wave guide switching device to vary the energy supplied to the cooking cavity depending upon the load installed therein. Each of these methods has certain drawbacks, particularly if installed in the hybrid oven described above.
In U.S. Pat. Nos. 5,254,823, 5,434,590 and 5,558,793. to McKee et al. ("the McKee Patents"), the contents of which are explicitly incorporated by reference herein, a hybrid oven is disclosed for cooking by both hot air impingement and microwave energy. Each oven includes an apparatus, which is shown in FIG. 1 and is generally designated by reference numeral 10, for illuminating a heating cavity with microwave energy. In FIG. 1, first magnetron 11 feeds microwave energy at a preselected frequency into first waveguide 12, and second magnetron 13 feeds microwave energy at the same preselected frequency into second waveguide 14. First waveguide 12 and second waveguide 14 each feed the respective microwave energy into the common third waveguide 15, which mixes the microwave energy and directs it into launcher 16. Launcher 16 is a cylindrical waveguide which directs the mixed microwave energy upward into the heating cavity (not shown) which is disposed directly above launcher 16. A mode stirrer (not shown) may be optionally installed within the launcher 16 to provide a more uniform pattern of illumination in the microwave energy supplied to the heating cavity. In practice, the conventional devices have certain drawbacks. In particular, the loading effects produced were found to be less than optimal.